FIG. 1(a) shows a circuit diagram of an equivalent circuit of a typical solar cell unit, wherein D is an LED, Rsh is a parallel-connected inner resistor, Rs is a series-connected inner resistor, and Iph is an output current of the solar cell unit. There are six well-known solar energy MPPT techniques including voltage feedback method, power feedback method, practical measurement method, linear approximation method, perturbation and observation method and incremental conductance method.
In these six methods, the perturbation and observation method is the most widely used one. This method uses the perturbation to measure the new output voltage and current of the two sides of the solar panel, calculate its power, and compare with the power sampled last time to get its change amount. If the new power value is higher than the power value of last time, it represents that the perturbation direction is correct. Otherwise, the direction of perturbation shall be reversed. And, the next movement of adding or subtracting the perturbation is decided accordingly. Since the procedure of perturbation will constantly change the output power of the solar panel (or the PV array), the last operating point would be stabilized within a small range around the maximum power point (MPP). The drawback of this method is that the procedure of perturbation will never be ceased, which will cause oscillation around the MPP, result in the energy loss and decrease the efficiency of conversion.
The incremental conductance method is applied via a principle that a rate of change of an output power with respect to a voltage of a solar panel is zero at an MPPT, and at a place corresponding to dP/dV=0 on the current-voltage characteristic curve, e.g. as shown in FIG. 1(b), and the incremental conductance method directly finds out
                                                        Δ              ⁢                                                          ⁢              I                                      Δ              ⁢                                                          ⁢              V                                =                      -                          I              V                                      ,                            (        1        )            
where I is a solar cell current, V is a solar cell voltage, ΔV is a voltage increment, and ΔI is a current increment. Via measuring a conductance value of ΔI/ΔV and compared it with an instantaneous conductance of −I/V of the solar panel to judge whether ΔI/ΔV is larger than, smaller than, or equivalent to −I/V so as to determine whether the next incremental change should be continued. When the incremental conductance conforms to formula (1), the solar panel is for sure to be operated at a maximum power point (MPP), and there will be no more next increment. This method engages in a tracking via the modification of the logic expression, there is not any oscillation around the MPP such that it is more suitable to the constantly changing conditions of the atmosphere. The incremental conductance method can accomplish the MPPT more accurately and decrease the oscillation problem as in the perturbation and observation method, but it still has some drawbacks. As shown in FIG. 1(b), using the curve of insolation 1000 W/m2 as an example, if it is detected at the very beginning that the solar panel is working at point A on the Ipv-Vpv curve, that is at the left-hand side of point C (the MPP), and corresponding to point A′ on the Ppv-Vpv curve, the (photovoltaic) system will cause the solar panel to move its operating point from A to its right-hand side and track towards the point C. On the contrary, if it is detected that the solar panel is working at point B on the Ipv-Vpv curve, that is at the right-hand side of point C (the MPP), and corresponding to point B′ on the Ppv-Vpv curve, then the system will cause the solar panel to move its operating point from B to its left-hand side and track towards the point C. Finally, when the detected conductance value of ΔI/ΔV satisfies formula (1), the system will keep the operating point of the solar panel at point C of the Ipv-Vpv curve and at point C′ of the Ppv-Vpv curve to maintain the maximum power output of the solar panel. However, the drawbacks of the incremental conductance method are that at the left-hand side of point C (the MPP) of the Ipv-Vpv curve, it is found that the change of current with respect to voltage is almost a constant value in most of the sections, and is approximately equal to the short-circuit current, and at the right-hand side of point C, the problem is the change of voltage with respect to current is not obvious. In other words, at the left-hand side of point C, the change of current with respect to voltage is relatively insensitive and has a poor responsibility. On the contrary, at the right-hand side of point C, the change of voltage with respect to current is relatively insensitive. However, the best tracking range of the incremental conductance method is in the area around point C, where the changes of current and voltage are relatively obvious, and it has a superior responsibility. But, on the left-hand side and the right-hand side of point C, there are respectively drawbacks of the change of current with respect to voltage is relatively insensitive and the change of voltage with respect to current is relatively insensitive such that result in the incremental conductance method could not give full scope of its function and there are phenomena of tardiness and slow response speed when engaged in the MPPT, which will influence the output efficiency of the maximum power.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a photovoltaic system having a power-increment-aided incremental-conductance maximum power point tracking controller using a variable-frequency constant-duty control and method thereof.